/Engine/Source/Runtime/GeometryCore/Public/MeshRegionBoundaryLoops.h
E needs to be large enough (ie call GetVtxBoundaryEdges, or as large as max one-ring) returns count, ie number of elements of e that were filled
int GetAllVertexBoundaryEdges ( int vID, [TArray](API\Runtime\Core\Containers\TArray)< int > & e )